In conventional processes for producing a polyester resin, a polycondensation reaction is usually performed in the presence of a metal-containing catalyst so as to enhance the reaction rate. However, the presence of the metal-containing catalyst may cause undesirable degradation of the polyester resin produced such that the polyester resin may have problems such as an insufficient molecular weight and an inferior color.
In order to solve the aforesaid problems, a phosphorus-containing additive (e.g., phosphoric acid and derivatives thereof, phosphorous acid and derivatives thereof, and the like) is usually used as a stabilizer in the production of the polyester resin to inhibit the degradation of the polyester resin. However, the reaction rate of the polycondensation reaction may thus be decreased and the yield of the polyester resin may also be reduced accordingly.
U.S. Pat. No. 6,384,180 B1 discloses a process for making a polyester resin employing an acidic phosphorus-containing compound, such as phosphoric acid, phosphorous acid, polyphosphoric acid, acidic phosphate esters or mixtures thereof. It is also disclosed therein that when phosphoric acid is added to an esterification product containing antimony, the finisher time is increased significantly with increasing phosphorus level, and certain inorganic compounds such as antimony phosphate may be produced to cause particulate haze in the polyester resin. Furthermore, use of a phosphate triester as a phosphorus additive is also disclosed therein.